This invention relates to a PEM fuel cell system having a substantially isothermal, thermally integrated CO-cleansing subsystem to optimize efficiency and reaction control.
PEM fuel cells have been proposed for many applications including electrical power plants to replace internal combustion engines. PEM fuel cells are well known in the art and include a xe2x80x9cmembrane electrode assemblyxe2x80x9d (a.k.a. MEA) comprising a thin, proton transmissive, solid polymer membrane-electrolyte having an anode on one of its faces and a cathode on the opposite face. The solid polymer electrolytes are typically made from ion exchange resins such as perfluoronated sulfonic acid. The anode/cathode typically comprise finely divided catalytic particles (often supported on carbon particles) admixed with proton conductive resin. The MEA is sandwiched between a pair of electrically conductive elements which (1) serve as current collectors for the anode and cathode, and (2) contain channels for distributing the fuel cell""s gaseous reactants over the surfaces of the respective anode and cathode. In PEM fuel cells, hydrogen is the anode reactant (i.e. fuel) and oxygen is the cathode reactant (i.e. oxidant).
For vehicular applications it is desirable to use a carbon-bound hydrogenous fuel (e.g. methane, gasoline, methanol, etc.). Liquid such fuels are particularly desirable as the source of the hydrogen used by the fuel cell owing to their ease of on board storage and the existence of a nationwide infrastructure of service stations that can conveniently supply such liquids. These fuels must be dissociated to release their hydrogen content for fueling the fuel cell. The dissociation reaction is accomplished in a so-called xe2x80x9cprimary reactorxe2x80x9d. One known such primary reactor for gasoline, for example, is a two stage chemical reactor often referred to as an xe2x80x9cautothermal reformerxe2x80x9d. In an autothermal reformer, gasoline and water vapor (i.e. steam) are mixed with air and pass sequentially through two reaction sections i.e. a first xe2x80x9cpartial oxidationxe2x80x9d (POX) section, and a second steam reforming (SR) section. In the POX section, and with or without a catalyst, the gasoline reacts exothermically with a substoichiometric amount of air to produce carbon monoxide, hydrogen and lower hydrocarbons such as methane. The hot POX reaction products, along with the steam introduced with the gasoline, pass into a SR section where the lower hydrocarbons react and a fraction of the carbon monoxide react with the steam to produce a reformate gas comprising principally hydrogen, carbon dioxide, nitrogen and carbon monoxide. The SR reaction is endothermic, but obtains its required heat either from the heat that is generated in the exothermic POX section and carried forward into the SR section by the POX section effluent, or from other parts of the fuel cell system (e.g. from a combustor). One such autothermal reformer is described in International Patent Publication Number WO 98/08771 published Mar. 5, 1998.
The carbon monoxide contained in the SR effluent must be removed, or at least reduced to very low concentrations (i.e. less than about 20 ppm) that are non-toxic to the anode catalyst in the fuel cell. It is known to cleanse the SR effluent of CO by subjecting it to a so-called xe2x80x9cwater-gas-shiftxe2x80x9d reaction (WGS) which takes place in a water-gas-shift reactor located downstream of the SR reactor. In the water-gas-shift reaction, water (i.e. steam) reacts exothermically with the carbon monoxide in the SR effluent according to the following ideal shift reaction:
CO+H2Oxe2x86x92CO2+H2
None-the-less, some CO still survives the water-gas-shift reaction and needs to be reduced further (i.e. to below about 20 ppm) before the reformate can be supplied to the fuel cell. It is known to further reduce the CO content of H2-rich reformate exiting a water-gas-shift reactor by reacting it with oxygen (i.e. as air) in a so-called xe2x80x9cPrOxxe2x80x9d reaction (i.e. preferential oxidation) carried out in a catalytic PrOx reactor. The PrOx reaction is exothermic and proceeds as follows:
CO+1/202xe2x86x92CO2
The PrOx reactor effluent (i.e. CO-cleansed, H2-rich reformate) is then supplied to the fuel cell. The PrOx reaction is also known as SelOx (i.e. selective oxidation).
Typical such fuel cell systems are thermally complex having a plurality of system components and working fluids (i.e. reactant streams such as fuel, air, reformate, etc.) all operating at different temperatures. Accordingly, such systems are often complex to control and slow to start-up after they have cooled down following a shutdown. The present invention simplifies the thermal management and start-up of PEM fuel cells fueled by hydrogen derived from carbon-bound hydrogenous fuels.
The present invention contemplates a PEM fuel cell system having an independent, substantially isothermal, heat transfer subsystem that communicates, and substantially thermally dominates, selected components of the fuel cell system. The isothermal heat transfer subsystem contemplated by this invention: (1) thermally integrates PrOx and water-gas-shift reactors that are designed to operate at substantially the same temperature, as well as other system components (e.g. heat exchangers) that operate at about that same temperature; (2) facilitates start-up of the PrOx and shift reactors without concern for over-heating or damaging the reactors"" catalysts; and (3) simplifies control of the reactors by imposing a reaction temperature thereon that is not appreciably affected by the heat generated by the reactions. By xe2x80x9cthermally dominatexe2x80x9d a component is meant a condition wherein the combination of the flow rate and the specific heat of the heat transfer medium used in the heat transfer circuit is such that the heat transfer circuit is the dominant or controlling factor effecting the operating temperature of that component.
According to a preferred embodiment, this invention involves a PEM fuel cell system that comprises a primary reactor that converts a carbon-bound hydrogenous fuel (e.g. gasoline) into a H2-rich reformate gas for fueling the fuel cell. The processor has (1) a first POX section in which the gasoline is reacted with a substoichiometric amount of oxygen to form a gas stream containing lower hydrocarbons (e.g. methane) and first concentrations of CO and H2, and (2) a second SR section, downstream of the first POX section, in which the gas stream exiting the POX section is catalytically reacted with steam to form a reformate gas having a second CO concentration that is less than the first CO concentration and a second H2 concentration that is greater than the first H2 concentration. The system also includes at least one water-gas-shift reactor downstream of the primary reactor that reacts a portion of the CO in the reformate gas exiting the primary reactor with steam to reduce the CO concentration in the fuel gas to a third CO concentration below the second CO concentration in the SR reactor effluent, and to increase the H2 concentration above the second H2 concentration in the SR reactor effluent. Multiple water-gas-shift reactors operating at different temperatures may be used in lieu of a single water-gas-shift reactor. Still further the system includes a PrOx reactor that selectively reacts some of the CO in the reform ate gas exiting the water-gas-shift reactor with oxygen (i.e. from air) to reduce the CO concentration in the reformate gas below the third CO concentration, and yield a CO-lean gas in which the CO concentration is non-toxic to the fuel cell.
The present invention contemplates such a PEM fuel cell system wherein: (1) the PrOx reactor is an isothermal reactor whose catalyst is selected to effect the selective oxidation of CO at a particular temperature; (2) the water-gas-shift reactor is also an isothermal reactor whose catalyst is selected to effect the water-gas-shift reaction at substantially the same temperature as the PrOx reaction occurs; (3) there is at least one heat exchanger that transfers heat either to or from at least one of the system""s working fluids; (4) there is a closed-loop heat transfer circuit that communicates with the water-gas-shift reactor, the PrOx reactor, and the heat exchanger(s); (5) there is a heat transfer liquid having a relatively high specific heat that circulates in the circuit through the water-gas-shift reactor, the PrOx reactor, and the heat exchanger (s); and (6) there is a pump that circulates the heat transfer liquid at a sufficiently high rate throughout the circuit as to maintain the water-gas-shift reactor, the preferential oxidation reactor, and the heat exchanger all at substantially the operating temperature selected for the PrOx reactor. For purposes of this application, the term xe2x80x9csubstantially the operating temperature of the PrOx reactorxe2x80x9d, as used herein, means a temperature that falls within about 20xc2x0 C.xc2x1 degrees of the operating temperature of the PrOx reactor. Other operating temperatures may be selected to optimize the over all volume and mass of the components in the heat transfer circuit. In one embodiment of the invention, the heat exchanger is a vaporizer that serves to vaporize (1) a carbon-bound hydrogenous liquid fuel (e.g. gasoline) before it is introduced into the fuel processor, or (2) water before it is introduced into either the primary reactor or the water-gas-shift reactor. In another embodiment, the heat exchanger is a heater for preheating the oxygen before it is introduced into the fuel processor. In a further embodiment, the heat exchanger is a cooler for cooling the reformate gas before it is introduced into the water-gas-shift reactor. In still another embodiment, the heat exchanger is a by-passable cooler (e.g. a radiator) for cooling the heat transfer liquid if its temperature is substantially greater than the operating temperature of the PrOx reactor. If the temperature of the heat transfer liquid is within acceptable limits (i.e. does not require cooling), this cooler may be bypassed using appropriate plumbing. In yet another embodiment, the heat exchanger is a heater that heats (e.g. with heat from a combustor) the heat transfer liquid if its temperature is substantially less than the operating temperature of the PrOx reactor.
According to a preferred embodiment of the invention, the heat transfer circuit will contain a plurality of heat exchangers each performing one or more of the heating/cooling functions set forth above. Most preferably, the heat transfer circuit will contain all of the heat exchangers set forth above.